The instant invention relates to dry, electrostatic copiers having a developer unit with a brush for depositing toner particles on the surface of a photoreceptor. More particularly, the invention relates to the mounting apparatus formed in such copiers for mounting the developer unit adjacent the photoreceptor.
Electrostatic copiers are known which employ a photoconductive element (photoreceptor), such as a zinc oxide coated web, for developing a visible image of a document to be reproduced. In such a device, a latent image is first formed on the surface of the photoreceptor by depositing electrostatic charges thereon and selectively dissipating certain charges to form a charge pattern corresponding to the original document. A dry, visible toner powder is then distributed onto the charged surface of the photoreceptor, customarily by means of a magnetic brush which rotates about an axis parallel to the transverse direction of the photoreceptor. By the mechanism of electrostatic attraction, the distributed toner particles adhere to the electrostatic charges on the photoreceptor, thereby rendering the latent image visible. This visible image is then ordinarily transferred to a second surface, such as a sheet of paper, and fixed thereon by heating.
While there are many determining factors which influence the quality of the developed image, given a substantially uniform surface charge distribution, the single most significant factor determining image quality is the degree of uniformity with which the toner powder is distributed on the surface of the photoreceptor. Many copiers employ magnetic brush development units for distributing the toner. In a typical arrangement, the brush consists of a magnetized core member, such as a permanently magnetized cylinder, and magnetic particles, such as iron filings, which are attracted by magnetic forces to the surface of the core, forming brushlike tufts or streamers on the surface thereof. The brush is ordinarily encased in a developer unit which also carries components for mixing the toner powder with the magnetic particles (two component toner). In operation, the toner powder is introduced into the mixing portion of the developer unit where it is thoroughly intermixed with the magnetic particles. During mixing, particles of toner are attracted by and adhere to the magnetic particles by triboelectric forces therebetween. The toner bearing magnetic particles are then swept over the charged photoreceptor by the brush core. Those toner particles which encounter electrostatic charges are removed from their magnetic carriers by the force of electrostatic attraction therebetween and remain on the photoreceptor. The magnetic carrier particles are then returned by the brush core to the mixing portion of the developer unit where they acquire fresh toner particles, and the process is repeated.
It has been found that optimum results are obtained if the photoreceptor is arranged vertically in the working region, defined by the charging and developing stations in those copiers which fix the visible image on the photoreceptor (copy paper), and defined by the charging and transfer stations in those copiers which fix the visible image after transfer to a second surface. However, this arrangement introduces a critical parameter in the design of the developing station: namely, the separation distance between the surface of the charged photoreceptor and the surface of the magnetic brush core. If this distance is too great, no toner particles will approach close enough to the surface charges on the photoreceptor to be attracted thereto and the result will be an undeveloped, invisible image. On the other hand, if this distance is too small, the surface friction between the magnetic particles and the photoreceptor surface will cause great quantities of the magnetic particles to be separated from the brush core, causing partial dissipation of the charge distribution (since the magnetic particles are also electrically conductive), fouling of machine components located below the developing station and an excessive deposition of toner particles onto the photoreceptor surface which results in an exceedingly dark and sometimes completely black developed image. In between these two extremes lies an optimum spacing which varies accordingly to the desired image contrast, the magnetic field strength at the brush core surface, the permeability and linear dimensions of the magnetic particles, the strength of the triboelectric forces between the toner and magnetic particles, and other factors. Once achieved, this optimum spacing must be rigidly maintained to ensure good copy quality.
The foregoing problems become even more acute with the use of a single component toner (so-called because there is no separate magnetic carrier particle), thereby making proper registration between the photoreceptor and the developer unit all the more critical. It is also desirable to be able to retract the developer unit to facilitate replacement of a photoreceptor belt, and to be able to completely remove the developer unit from the copier housing. The foregoing objectives are all achieved by the instant invention, which provides for alignment of the developer unit in three mutually perpendicular directions and facilitates removal of the developer unit from the copier housing.